The present invention relates to the production of synthetic iron based pigments and anticorrosive additives.
Conventional chemical methods for the production of synthetic iron oxide pigments are Known (See. e.g. T. C. Patton: xe2x80x9cPigment Handbook 1988. vol.1. Second Edition. Properties and Economics, pp. 297-302xe2x80x9d. U.S. Pat. Nos. 4,911,760, 5,268,640, 3,969,494, 5,185,141, 4,349,385; O. V. Orlova and T. N. Fomitcheva:xe2x80x9d Paints and Lacquers Technology, 1990, Moscow, xe2x80x9cChemistryxe2x80x9d, pp. 294-302). As a raw materials for such production, there is used ferrous sulfate (FeSO47H2O), sodium hydroxide (NaOH) and very high quality iron filings (Fe0). In the first step of this method, the ferrous sulfate is reacted with sodium hydroxide in order to produce a ferrous hydroxide (Fe (OH)2 suspension. The second step is the oxidation of Fe(OH)2 by air at a temperature of about 85-90xc2x0 C. in the presence of about 10% of the separately prepared nuclei suspensions of goethite (xcex1-FeOOH).
During the oxidation, Fe(OH)2 transforms into xcex1-FeOOH yellow iron monohydroxide pigment. At this time of oxidation, the solution""s pH changes from about 9.5-10.0 to about 4.0-4.5. In order to prevent the pH decrease and precipitation of ferric sulfate basic salts, it is necessary to continuously add high quality iron filings into the solution. In this manner, elementary iron reacts with the formed sulfuric acid and prevents the decrease in the solution""s pH. The complete oxidation of Fe2+ into Fe3+ takes place in about 5-18 hours. The third step of this method is to allow for the growth of pigment particles according to predetermined standards. Thereafter, the pigment is separated from the solution (Na2SO4) by filtration, washed and dried. As a result of this chemical method, there is produced a yellow iron oxide pigment with particle size of about 0.1-0.3xc3x970.5-0.9 microns. The pigment particles have needle or prismatic form. Mineralogically, this is characterized as a goethite (xcex1-FeOOH) form. The by-product of this process is sodium sulfate. In order to produce a red iron oxide, it is necessary to calcine the produced yellow iron oxide at a temperature of about 400-450xc2x0 C. Red iron oxide is hematite (Fe2O3).
The disadvantages of the existing chemical technology of the aforementioned yellow iron oxide pigment production are:
The need for high quality initial raw materials since at high alkaline conditions of reaction almost all of the metals accompanying iron production are precipitated and pollute the iron oxide pigment.
The need for a special nucleuses suspension""s preparation which is an independent and expensive technological operation, including Fe (OH)2 complete oxidation and transformation into xcex1-FeOOH at the temperature of about 85-90xc2x0 C.
Long process duration of Fe(OH)2 oxidation, utilizing significant quantities of required air and requiring the maintenance of a high solution temperature.
Necessity of the constant use of the extremely pure metal iron: for the pigment quality""s stabilization, for the maintenance of pH at a determined level and for the prevention of the basic iron sulfate salts"" precipitation together with pigment.
Formation of needle or prismatic forms of particles, worsening the pigment""s properties.
Na2SO4 formed as a result of the reaction as a by-product is a practically useless, cheap product and its pollution is harmful for the environment.
The present invention provides a process for the preparation of yellow and other iron-based pigments which does not suffer from the above-mentioned drawbacks. It is much cheaper, ecologically clean and allows to produce very high quality yellow and red iron oxide pigments from low quality initial raw material such as iron waste. At the same time the present process enables the production of other iron-based pigments and anticorrosive additives, such as a black iron oxide pigment (magnetite), Prussian blue and ferric monophosphate (anticorrosive additive) with a minimal added expense.
Thus, according to the present invention, there is now provided a process for the production of iron based pigments and anticorrosive additives from elemental iron comprising:
a. reacting iron Fexc2x0 with ferric sulphate Fe2(SO4)3 and with sulfuric acid H2SO4 to produce ferrous sulphate FeSO4:
b. oxidizing ferrous sulphate FeSO4 in a bacterial solution containing sulfuric acid and at least 107 bacterial cells of thiobacillus ferrooxidans per 1 ml of solution under aerobic conditions to produce ferric sulphate Fe2(SO4)3:
c. reacting said ferric sulphate Fe2(SO4)3 with water at a temperature of about 65 to 130xc2x0 C., to precipitate iron oxide monohydrate Fe2O3xc3x97H2O; and
d. re-cycling the solution from step c, to step a, to utilize remaining sulphuric acid and a portion of unhydrolyzed ferric sulphate values therein.
As an initial raw material, there is preferably used iron waste or iron filings, containing from 40 to 100% of elementary iron.
Preferably, the process proceeds at ambient (10-35xc2x0 C.) temperature in aerobic acidic (pH 1.0-2.9) sulfate environments, at the concentration of iron in working solutions from less 10 up to 40 g/l of iron. The biochemical reaction of ferrous sulfate""s oxidation is shown below:
2FeSO4+2H2SO4+O2+Th.ferrooxidans=2Fe2(SO4)3+2H2O
Produced by bacterias the ferric sulfate solution is used as an oxidizer and further its portion is used for primary iron oxidation and production of the ferrous sulfate:
Fe+Fe2(SO4)3=3FeSO4
In the proposed biotechnical reaction, the iron""s oxidation speed depends on the quantity of bacteria cells in the solution. According to the present invention, it was established, that for the achievement of the economically favorable oxidation""s speed (about 1 g of iron in 1 liter of a solution during 3 hours) it is necessary to have no less than 107 bacterias cells in 1 ml of the solution. At such concentration of the cells a ratio between incoming ferrous sulfate solution and bacterial suspension of the fermenter is preferably about 1:10. The increase of this ratio for the benefit of the ferrous sulfate solution delays the oxidation""s speed. A lack of the ferrous sulfate feed brings to the decrease of the bacterias cells and premature bacterias decease.
During the bacterias vital activitys the resulting solution consists of three main components, i.e. sulfuric acid, ensuring of the solution""s pH in the interval 2.1-2.8, colloidal particles of iron hydroxide Fe (OH)3, formed as a result of ferric sulfate partial hydrolysis, and unhydrolyzed ferric sulfate. This product is the initial raw material for the production of all above mentioned pigments and anticorrosive additives, e.g. yellow (xcex1-FeOOH), red (Fe2O3), black (Fe3O4), Prussian blue (Fe4(Fe(CN)6)3) and ferric dihydrogen phosphate (Fe(H2PO4)3).
Before its further use the solution is separated from bacteria by filtration, and the separated bacteria is recycled into the fermenter. About 30% of iron is in a colloidal form as a phase of Fe (OH)3, formed as a result of the partial ferric sulfate hydrolysis which step by step takes place within the fermenter in the presence of a surplus of water, according to the reaction:
Fe2(SO4)3+6H2O=2Fe(OH)3+3H2SO4
The form of the colloidal particle is spherical. Therefore, if the pigment""s production occurs without additional ferric sulfate hydrolysis, formed xcex1-FeOOH has the inherited spherical form of particles. An additional hydrolysis of the remaining ferric sulfate results in the formation of the particles in form of needles. Therefore it is possibility to produce the yellow iron oxide pigment with only spherical particles or as a mixture of spherical and needle particles.
Depending on the requirement, the solution produced by the bacteria treatment can be used as follows:
a) For the manufacture of yellow iron oxide pigment this solution is exposed to treatment at a temperature range of about 65-130xc2x0 C. for about 1.5-2 hours. As a result of this treatment the colloidal solution of Fe(OH)3 coagulates and transforms into xcex1-FeOOH (goethite). Thus the solution""s pH is reduced from 2.10-2.8 to 1.5-1.8[.] Other metals, if they also contain a solution, cannot precipitate because at the given interval of pH only the ferric sulfate is hydrolyzed. After treatment at a temperature range of about 65-130xc2x0 C. for about 1.5-2 hours the precipitated yellow iron oxide pigment is separated from the solution with filtration, washed until a pH of about 5.5-6.0 is reached and dried. The yellow iron oxide pigment produced consists of spherical, or spherical and needle particles which have the size of about 20-120 nm. After the filtration, there remains a solution of unhydrolyzed ferric sulfate, sulfuric acid, and also water with a pH of about 1.5-1.8 which is recycled to the beginning of the process for participation in the oxidation of the iron filings and in order to produce again the ferrous sulfate solution.
b) For the manufacture of iron oxide pigment (Fe2O3) there can be used the manufactured yellow iron oxide pigment. For this purpose the yellow pigment is calcined at the high temperatures of about 400 up to 800xc2x0 C. The calcination""s temperature depends on the red pigment shade;
c) For the manufacture of the black iron oxide pigment (Fe3O4) they are used two solutions: ferrous sulfate and ferric sulfate. The first solution is taken from the first step of iron oxidation and the second solution is taken from the fermenter. After mixing, the ratio of Fe2+ and Fe3+ should be 1:2 as in a magnetite. The prepared ferrous and ferric sulfate mixture is reacted with a stoichiometric quantity of the ammonium hydroxide. This reaction takes place at ambient temperature according to the following reaction scheme:
Fe2(SO4)3+FeSO4+4NH4OH=Fe3O4+4(NH4)2SO4+4H2O
Instead of ammonium hydroxide any other alkali can be used. After reaction the precipitated black iron oxide is separated from the solution with filtration, washed at a pH of about, 6-7 and dried. The product is in the form of spherical particles with sizes from about 20 up to 150 nm. The ammonium sulfate solution remaining after filtration is condensed, crystallized and separately recovered.
d) For the manufacture of Prussian blue pigment there is used the completely oxidized solution from the fermenter which is reacted with a stoichiometric quantity of potassium ferrocyanide (yellow) at ambient temperature according to the following reaction scheme,
2Fe2(SO4)3+3K4(Fe(CN)6)=Fe3(Fe(CN)6)3+6K2SO4
As a result of the chemical reaction, Prussian blue pigment is precipitated. The precipitate is separated from solution with filtration, is washed and dried. As a by-product in this case, potassium sulfate is produced. It can be condensed, crystallized and separately recovered and used as a fertilizer.
e) For the manufacture of ferric phosphates anticorrosive additives there is used the completely oxidized solution from the fermenter at ambient temperature. This solution is reacted with a stoichiometric quantity of the potassium dihydrogen phosphate according to the following reaction scheme:
Fe2(SO4)3+2KH2PO42Fe(H2PO4)3+K2SO4
As a result of the chemical reaction a white precipitate of ferric dihydrogen phosphate is formed. This precipitate is filtered, washed and dried. As a by-product in this case potassium sulfate solution is produced. It can be condensed, crystallized and separately recovered and used as a fertilizer.
In British Patent 1,245,169 there is described and claimed a process for converging elemental iron and ferrous iron to high purity ferric oxide which comprises cultivating an iron-oxidizing bacterium at a pH of from 2.2 to 3 and a temperature of from 15 to 40xc2x0 C. under aerobic conditions in an aqueous nutrient medium containing elemental iron and ferrous sulfate equivalent to at least one gram per liter of ferrous iron, and recovering the hydrous ferric oxide which precipitates. Said Patent, however, results in the precipitation of Fe(OH)3 and as explained hereinbefore and does not teach or suggest the step of reacting said ferric sulphate Fe2(SO4)3 with water at a temperature of about 65 to 130xc2x0 C., to precipitate iron oxide monohydrate Fe2O3xc3x97H2O; and which results in the production of very pure crystalline and nannometric goethite (FeOOH) particles.
While the invention will now be described in connection with certain preferred embodiments in the following examples so that aspects thereof may be more fully understood and appreciated, it is not intended to limit the invention to these particular embodiments. On the contrary, it is intended to cover all alternatives modifications and equivalents as may De included within the scope of the invention as defined by the appended claims. Thus, the following examples which include preferred embodiments will serve to illustrate the practice of this invention it being understood that the particulars shown are by way of example and for purposes of illustrative discussion of preferred embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of formulation procedures as well as of the principles and conceptual aspects of the invention.